1. Field of the Invention
Embodiments of the invention generally relate to a gas distribution plate assembly and method for distributing gas in a processing chamber.
2. Description of the Background Art
Liquid crystal displays or flat panels are commonly used for active matrix displays such as computer and television monitors. Generally, flat panels comprise two glass plates having a layer of liquid crystal material sandwiched therebetween. At least one of the glass plates includes at least one conductive film disposed thereon that is coupled to a power supply. Power supplied to the conductive film from the power supply changes the orientation of the crystal material, creating a pattern such as text or graphics seen on the display. One fabrication process frequently used to produce flat panels is plasma enhanced chemical vapor deposition (PECVD).
Plasma enhanced chemical vapor deposition is generally employed to deposit thin films on a substrate such as a flat panel or semiconductor wafer. Plasma enhanced chemical vapor deposition is generally accomplished by introducing a precursor gas into a vacuum chamber that contains a flat panel. The precursor gas is typically directed downwardly through a distribution plate situated near the top of the chamber. The precursor gas in the chamber is energized (e.g., excited) into a plasma by applying RF power to the chamber from one or more RF sources coupled to the chamber. The excited gas reacts to form a layer of material on a surface of the flat panel that is positioned on a temperature controlled substrate support. In applications where the flat panel receives a layer of low temperature polysilicon, the substrate support may be heated in excess of 400 degrees Celsius. Volatile by-products produced during the reaction are pumped from the chamber through an exhaust system.
Flat panels processed by PECVD techniques are typically large, often exceeding 370 mm×470 mm and ranging over 1 square meter in size. Large area substrates approaching and exceeding 4 square meters are envisioned in the near future. Gas distribution plates utilized to provide uniform process gas flow over flat panels are proportionally large in size, particularly as compared to gas distribution plates utilized for 200 mm and 300 mm semiconductor wafer processing.
Large gas distribution plates utilized for flat panel processing have a number of fabricating issues that result in high manufacturing costs. For example, gas flow holes formed through the gas distribution plate are small in diameter relative to thickness of the gas distribution plate, for examples a 0.062 diameter hole through a 1.2 inch thick plate, resulting in a high frequency of drill bit breakage during hole formation. Removal of broken drill bits is time consuming and may result in the entire gas distribution plate being scrapped. Additionally, as the number of gas flow holes formed through the gas distribution plate is proportional to the size of the flat panel, the great number of holes formed in each plate disadvantageously contributes to a high probability of trouble during plate fabrication. Moreover, the high number of holes coupled with the care required to minimize drill bit breakage results in long fabrication times, thereby elevating fabrication costs.
As the cost of materials and manufacturing the gas distribution plate is great, it would be advantageous to develop a gas distribution plate in a configuration that can be efficiently and cost effectively fabricated. Moreover, as the size of the next generation gas distribution plates is increased to accommodate processing flat panels in excess of 1.2 square meters, resolution of the aforementioned problems becomes increasingly important to resolve.
While addressing the cost implications of the design of large gas distribution plates is important, performance attributes must not be overlooked.For example, the configuration, location and density of gas flow holes directly impact deposition performance, such as deposition uniformity and cleaning attributes. For example, if the gas flow holes formed through the gas distribution plate create too much backpressure, disassociated fluorine utilized to clean the plate has an increased propensity to recombine, disadvantageously diminishing cleaning effectiveness. Moreover, as fluorine is typically a film contaminant, the surface area of the gas distribution plate should be configured to encourage good flow therethrough while providing minimal area for fluorine adherence to the plate.
Therefore, there is a need for an improved gas distribution plate assembly.